1. Field of the Invention
The present invention relates to a novel sulfonate and its derivative advantageously usable as a photosensitive acid generator and the like in a resist composition, a photosensitive acid generator, a resist composition using the same, and a patterning process.
2. Description of the Related Art
As LSI progresses toward a higher integration and a further acceleration in speed, a finer pattern rule is being requested. In such a movement, a deep-ultraviolet lithography is drawing an attention as the promising next-generation fine processing technology.
In recent years, technologies utilizing a KrF excimer laser having a high brightness and an ArF excimer laser having a further shorter wavelength have been drawing an attention as the light sources of a far ultraviolet ray. In addition, an ArF immersion lithography, which can be designed to have 1.0 or more of the numerical aperture (NA) of a projection lens by inserting a liquid having a higher refractive index than that of an air, such as water, ethylene glycol, and glycerin between the projection lens and a wafer, thereby attaining a high resolution, is rapidly drawing a growing attention (see, for example, Journal of photopolymer Science and Technology Vol.17, No. 4, p 587 (2004)). A further fine processing technology is sought by shifting toward a shorter wavelength of the exposure light and by attaining a higher resolution in the resist composition.
From this view point, a chemically amplified resist composition catalyzed by an acid, which has been developed recently, has excellent properties in sensitivity, resolution, and dry-etching resistance, and thus is a promising resist composition, particularly for a deep-ultraviolet lithography. In this chemically amplified resist composition, there are a positive type in which an exposed area is removed with leaving an unexposed area unremoved and a negative type in which an unexposed area is removed with leaving an exposed area unremoved.
In a chemically amplified positive resist composition using an alkaline developer, a part or all of a resin and/or a compound whose alkaline-soluble phenol group or carboxylic acid group is protected by an acid-unstable protection group (an acid-labile group) is catalytically decomposed, by an acid generated by an exposure, to form a phenol or a carboxylic acid in the exposed area, thereby removing this exposed area by an alkaline developer. On the other hand, in a chemically amplified negative resist composition, a resin and/or a compound having an alkaline-soluble phenol or carboxylic acid is crosslinked, by an acid generated by an exposure, with a compound (acid-crosslinker) that can link (crosslink) the resin or the compound by the acid to insolubilize the exposed part in an alkaline developer, thereby removing the unexposed part by the alkaline developer.
In the chemically amplified positive resist composition, a base resin having the acid-labile group and a compound generating the acid by radiation irradiation (hereinafter referred to as an photosensitive acid generator for short) are dissolved in a solvent, and the resist solution thus prepared is applied on a substrate by various ways, heated if necessary and then the solvent is removed, to form a resist film. Subsequently, the formed resist film is exposed to a light source such as a far ultraviolet ray by radiation irradiation through a prescribed mask pattern. Further, as appropriate, a post exposure bake (PEB) is done after the exposure to carry out an acid-catalyzed reaction, and the development by an alkaline solution is done to remove the exposed area of the resist film to obtain a positive pattern profile. After the substrate is etched by various ways, the remaining resist film is removed by dissolving in a delaminating solution or by ashing to form a pattern profile on the substrate.
In the chemically amplified positive resist composition for the KrF excimer laser, a resin whose part or all of hydrogen atoms of the phenolic hydroxyl group of a phenolic resin such as polyhydroxy styrene is protected by an acid-unstable protection group is used, and a photosensitive acid generator such as an iodonium salt, a sulfonium salt, a bissulfonyl diazomethane, an N-sulfonyloxy dicarboxyimide compound, an O-arene sulfonyloxime compound has been used. In addition, as appropriate, a dissolution inhibitor and/or a dissolution facilitator having a molecular weight of 3,000 or less formed of a carboxylic acid and/or a phenolic derivative wherein a part of or all of the hydrogen atoms of a carboxylic acid and/or a phenolic hydroxy group is protected by the acid-labile group, a carboxylic acid compound to improve dissolution characteristics, a basic compound to improve a contrast, a surfactant to improve coating characteristics, and the like are added.
Here, a photosensitive acid generator generating 10-camphor sulfonic acid or 2,4,6-triisopropylbenzene sulfonic acid suppresses diffusion not only in a sulfonium salt and an iodonium salt but also in an O-arene sulfonyloxime compound, and thus it is extremely useful for a high resolution resist composition (Japanese Patent Laid-Open No. H05-222257, Japanese Patent Laid-Open No. H10-39500, Japanese Patent Laid-Open No. 2004-133393, and Japanese Patent Laid-Open No. H09-323970). These photosensitive acid generators have a bulky structure thereby suitably suppressing an acid diffusion, and it is assumed that because of this an excellent resist performance is realized.
However, as a further miniaturization of the pattern size is required, there appeared problems of a low resolution, a poor environmental stability, and the like even when these photosensitive acid generators are used. Under such a circumstance, as to the resolution problem, an improvement is being made by making an acid-labile group in the used resin further easily breakable by an acid, and by using a basic additive and selecting a process condition.
The problems in the environmental stability may be classified roughly into two kinds; the one is the problem that the acid generated by light-exposure is inactivated by a base in an air above the resist film and on the substrate underneath the resist film, which is the phenomenon appears often when a photosensitive acid generator generating a highly acidic acid is used. The other problem of the environmental stability is that a generated acid diffuses in the resist film when the time of the light-exposure and the post exposure baking (PEB) after the exposure is prolonged, thereby inactivating the acid in the case when the acid-labile group is not easily breakable or facilitating the acid decomposition reaction in the case when the acid-labile group is easily breakable, and these, in turn, cause change of a pattern profile often. For example, narrowing of the line width of the unexposed area occurs often in the case of a chemically amplified positive resist composition having an acid-labile group such as an acetal group.
In Japanese Patent No. 3613491, an anion-bound PAG polymer is disclosed in a combination with a monomer having an acid-unlabile group. In this case, the effect as PAG is reduced due to the monomer having an acid-unlabile group, and thus resolution and the like are not sufficient.
As discussed above, in order to have an even higher resolution, introduction of a more easily breakable acid-labile group into a resin and use of a relatively weak acid as the photosensitive acid generator are necessary. However, the acid-labile group designed to be more easily breakable has a problem in the storage stability. When an acid-labile group having a good storage stability and an appropriate breakability is introduced, the resolution is insufficient in a further miniaturized patterning by a photosensitive acid generator generating a weak acid such as the 10-camphor sulfonic acid or 2,4,6-triisopropylbenzene sulfonic acid. On the other hand, use of a photosensitive acid generator generating a highly acidic acid such as an α-fluoroalkane sulfonic acid causes an environmental problem. Accordingly, a photosensitive acid generator is required to generate an acid having an appropriate acid strength.
The foregoing is essential also as a mask patterning method especially in an electron beam lithography, which is drawing an attention as an ultra-miniaturization process technology with 0.1 μm or less dimension.
However, drawing by an electron beam takes more time as compared with the conventional one-time light exposure. Accordingly, an even higher sensitivity is required to increase a throughput. In addition, a temporal stability under vacuum during and after the drawing is required as one of its important performances. Furthermore, depending on the substrate, some of a covering layer (such as SiO2, TiN, and Si3N3) on a silicon wafer, chromium oxide on a mask blanks, and the like, affect the resist form (footing profile) after development. Accordingly, in order to have the high resolution and maintain the form after etching, to keep a rectangular form of the resist pattern profile independent of a kind of the substrate is also one of its important performances.